1. Field of the Invention
The present invention relates to a driver IC for electrical loads which supplies electric power through an arrangement of current sources, and more particularly, to a driver IC for electrical loads which can sample, hold and feed back a lowest voltage among voltages dropped at both ends of respective current sources when a power supply voltage is supplied to electrical loads connected in parallel, thereby supplying a regulated power supply voltage, and a driving method thereof.
2. Description of the Related Art
So far, a load driving device for driving electrical loads such as light emitting diodes (LED) by using a DC-DC conversion type power circuit, which generates a voltage different from a power supply voltage, has been used.
The power circuit generates predetermined output voltage and output current, and the load driving device for driving the electrical loads detects an input voltage or a supply voltage applied to the electrical loads or current flowing through the electrical loads and feeds back the voltage or current to a control unit for the power circuit. Thereafter, the feedback voltage or load current is compared with a reference value, and the output voltage or the output current from the power circuit is controlled in such a way as to be regulated.
In a portable electronic appliance such as a mobile phone and the like, it may be necessary to increase or decrease load current for driving loads, as the occasion demands. That is to say, in the case where the loads are light emitting elements, it may be necessary to regulate light emissions of the light emitting elements in such a way as to have optional values.
In this case, loss increases due to the presence of resistors for current detection as load current increases, and accordingly, a problem is caused in that the entire efficiency of an electronic appliance including a power circuit or loads is likely to deteriorate.
FIG. 1 is a view illustrating a conventional power supply apparatus for operating electrical loads.
Referring to FIG. 1, the conventional power supply apparatus for operating electrical loads includes current sources 1, 11 and 21, means 2, 12 and 22 for connection of light emitting diodes 3, 13 and 23, voltage tapping nodes 4, 14 and 24, comparators 5, 15 and 25, downstream transistors 7, 17 and 27, and a DC voltage regulator 10.
In the conventional power supply apparatus for operating electrical loads, the current sources 1, 11 and 21 are arranged in such a way as to supply power to the one or more light emitting diodes 3, 13 and 23 which are connected in parallel.
When a power supply voltage VDD is supplied to the one or more light emitting diodes 3, 13 and 23 which are connected in parallel, in order to regulate the power supply voltage VDD, voltages dropped due to the presence of the respective light emitting diodes 3, 13 and 23 are measured. Such dropped voltages are compared with a threshold voltage Vt.
As a result of the comparison, output signals of the transistors 7, 17 and 27 are generated. In the case where the dropped voltages are smaller than the threshold voltage Vt, the transistors 7, 17 and 27 are turned on, by which a feedback voltage supplied through a common signal line 8 is dropped.
At this time, the DC voltage regulator 10 performs a boosting operation and increases the power supply voltage VDD as an output voltage in such a manner that a minimum power supply voltage supplied to the light emitting diodes 3, 13 and 23 is maintained.
However, in the case where the dropped voltages are larger than the threshold voltage Vt, while the DC voltage regulator 10 does not perform a boosting operation, a voltage no smaller than a necessary minimum voltage is applied to the current sources 1, 11 and 21, by which power loss is caused. Also, due to such unnecessary power consumption, a problem occurs due to generation of heat.